1. Field of the Invention
The present invention relates to a process for producing silicone carbide particles suitable for making a dense silicon carbide sinter. More specifically, it relates to a process for producing silicon carbide particles in which a free carbon content in the silicon carbide particles is controlled by monitoring a by-product such as acetylene. The present invention also relates to a process for producing a silicon carbide sinter from silicon carbide particles obtained by the above process.
2. Description of the Related Art
Silicon carbide has attracted attention as an excellent high temperature strength material suitable for use in gas turbines, etc. It is however difficult to sinter a silicon carbide body to a density close to the theoretical density of 3.21 g/cm.sup.3. Processes for obtaining a dense silicon carbide sinter have been proposed, for example, U.S. patent application Ser. No. 409,073 filed on Oct. 24, 1973, in which a boron-containing compound in an amount corresponding to 0.1 to 3.0 wt % of boron and a carbon source corresponding to 0.1 to 1.0 wt % of elemental carbon as densifying agents are dispersed uniformly with submicron .beta.-type silicon carbide particles, the resultant uniform dispersion is formed into a shape, and the shape is fired to obtain a dense silicon carbide sinter. To produce submicron .beta.-type silicon carbide particles, a gaseous trichloromethylsilane and hydrogen, or a suitable gaseous hydrocarbon such as silicon trichloride or toluene and hydrogen, are introduced into an argon plasma generated between two concentric electrodes to produce silicon carbide crystallites having a size of 0.1 to 0.3 .mu.m. It is also disclosed that free carbon may be contained in silicon carbide particles by using the carbon source in an amount slightly larger than the stoichiometric amount necessary for producing silicon carbide.
It is known that, to obtain an excellent silicon carbide sinter, silicon carbide particles containing boron and free carbon, desirably uniformly dispersed in the particles, as densifying agents, are preferable to a mixture of silicon carbide particles and densifying agents such as a boron source and a carbon source. The former allows a uniform structure of a sinter and an improvement of the mechanical properties of a sinter. Thus, the above U.S. patent application Ser. No. 409,073 discloses that free carbon may be contained in silicon carbide particles in a process for synthesizing the silicon carbide particles.
The amount of densifying agent is critical to the characteristics of a sinter. If the amount of densifying agent is not appropriate, a good quality sinter cannot be obtained. Therefore, when the free carbon produced in silicon carbide particles during the synthesis of silicon carbide particles is used as a densifying agent, control of the amount of the free carbon produced in the silicon carbide particles is very important.
It is, however, impossible to concurrently determine a quantitative amount of free carbon produced during the synthesis of silicon carbide particles. Thus, the amount of free carbon in silicon carbide particles is analyzed after a certain amount of silicon carbide particles have been synthesized to determine whether or not the amount of free carbon produced during the synthesis is appropriate. Further, to control the amount of free carbon produced in silicon carbide particles in accordance with the results of the above analysis, it is necessary to preliminarily examine the relationships between the reaction conditions and the amount of free carbon, by synthesizing silicon carbide particles under various reaction conditions. This necessitates a large number of experiments. Here, the reaction conditions include temperature, pressure, feed of raw materials, shape of a reaction chamber, etc.
Even if the reaction is conducted in specified conditions, the amount of free carbon in the silicon carbide particles may vary with the amount of time lapsed, because silicon carbide particles, etc., are deposited on the inside wall of a reaction chamber and, as a result, the residence time of a raw material in the reaction chamber and other factors are varied. This makes it difficult to produce free carbon in a desired amount.
U.S. patent application Ser. No. 471,303 filed on May 20, 1974 discloses a process for producing particles comprising a uniform dispersion of .beta.-type silicon carbide, boron and free carbon, in which a gaseous mixture essentially consisting of a silicon halide, a boron halide, and a hydrocarbon is introduced to a plasma jet reaction zone. This application suggests the effectiveness of a concurrent addition of boron halide during the synthesis of silicon carbide particles.
If a silicon compound, a carbon compound, and a boron compound are concurrently introduced into a single high temperature reaction zone, as in the U.S. patent application Ser. No. 471,303, mainly, silicon carbide is grown onto silicon carbide seeds, to give submicron silicon carbide crystals, and boron carbide or boron is grown onto boron carbide or boron seeds to give submicron boron carbide or boron crystals, respectively. Almost all of the boron is not doped in silicon carbide particles. In other words, high temperature stable extremely small seed crystals of silicon carbide, boron carbide and boron are first produced respectively, and then silicon carbide, boron carbide and boron are grown around these seed crystals, respectively, maintaining the same crystal structure, resulting in the formation of submicron silicon carbide, boron carbide and boron particles, respectively. In such a case, boron is not always contained in silicon carbide particles but tends to form boron carbide particles or boron particles, although a silicon compound, a carbon compound and a boron compound are added in a single reaction zone. Thus, almost all of the boron is not dispersed uniformly in the silicon carbide particles and it is difficult to obtain silicon carbide particles with boron uniformly dispersed therein.